1. Field of the invention
The present invention relates generally to the ground voltage bouncing problem in electronic devices, and more particularly, to the ground bouncing problem due to the current discharge from the capacitive load when the output is switching.
2. Description of Related Art
The ground bouncing phenomenon is a voltage ground Vss fluctuation induced by switching of the outputs of an electronic device. The bouncing is exacerbated by multiple outputs simultaneous switching and may cause malfunction of circuits having Vss as a reference ground. FIG. 1 depicts a typical device circuit with only one detailed output stage model shown being coupled to a capacitive load. A typical electronic device, such as integrated circuits (IC), comprises 8, 16, 32 or even 64 shown output stages. As shown in the FIG. 1, the output signals 12 and 13 of the circuit 11 are coupled to the gate terminals of the MOS transistors M1, M2, respectively. The drain terminal of the MOS transistor M1 is coupled to the power supply Vcc through an inductance L3 and the source terminal of the MOS transistor M2 is coupled to the ground voltage through an inductance L2. The inductance L2 is associated with the inductance inherently existing on the wiring between the source terminal of transistor M2 and the output pin of the IC package. An output 14 is taken at the connection of the source terminal of transistor M1 and the drain terminal of transistor M2. The capacitance C1 is associated with the loading to which the output is coupled and the inductance L1 is a wiring inductance. A voltage Vss is taken at the source terminal of the transistor M2 to be utilized as a ground reference to the circuit 11. For example, the internal flip-flops of the circuit 11 need a reliable and predictable ground reference Vss for accurate operation. When output signal 14 is not switching, or not changing state, no current is flowing through the inductance L2 and the voltage of Vss is equal to GND. However, as the output signal 14 is switching from high to low in response to the state change of signals 12 and 13, a current I is quickly discharging from the capacitive load C1 through inductance L1, MOS transistor M2 and inductance L2 to GND. The instantaneous value of the current I is equal to c1*dV/dt. The instantaneous value of the Vss induced is, therefore, equal to L2*dI/dt. The voltage induced on the source terminal of transistor M2 results in a ground voltage bouncing of Vss, and is known as the ground bouncing phenomenon in the art. The voltage induced at Vss will be substantial when a multiple of outputs are switching at the same time. For instance, as sixteen outputs switches from high to low voltage at the same time, the induced voltage is equal to 16*L2*dI/dt. A significant fluctuation of the Vss beyond a tolerance limit will cause malfunction of the circuits having Vss as a reference ground.